Advanced optical signatures of single, wurtzite GaN quantum dots : from fundamental exciton coupling mechanisms towards tunable photon statistics and hybrid-quasiparticles

University of Technology Sydney. Faculty of Science.The present work treats the fundamental optical signatures of individual, hexagonal GaN quantum dots embedded in AlN. The conducted experiments established the basis for numerous, novel observations, which are not only of interest for this particular quantum dot system, but also of general value for the entire quantum dot community. The presented analysis of the interaction in between quantum dot excitons and charged defects, as well as phonons, culminates in the first-time demonstration of quantum-optical device concepts for the ultraviolet (UV) spectral range. Here, a highlight is constituted by an optically pumped two-photon source based on the biexciton cascade, which maintains its highly promising photon statistics up to a temperature of 50K. Further in-detail studies of this biexciton cascade even lead to the description of so-called hybrid-quasiparticles in this work, with prominent consequences for a wide range of exciton-based quantum light sources. The first part of this thesis is dedicated to the preparation of multiexcitonic states. Based on the conjunction of excitation power dependent and time-resolved micro-Photoluminescence, an entire zoo of multiexcitonic complexes is identified for the first time. Here, the determination of relaxation times presents an observation with direct consequences for applications. Furthermore, it is demonstrated that the initial carrier capture process is predominantly realized by Auger-processes that dominate any multi-phonon contributions. However, in terms of intra-quantum dot carrier relaxation, it is exactly these multi-phonon processes that present the limiting factor, a phenomenon known as the “phonon-bottleneck” effect. As the emission of these excitons in hexagonal GaN is affected by “spectral diffusion”, a strong emission line widths broadening occurs, which still limits future applications but also any more fundamental analysis. A line width statistic is obtained by analyzing hundreds of individual GaN quantum dots, allowing an indirect determination of the average, defect-induced electric field, whose fluctuations originate the line widths broadening. A continuative statistical analysis is given for the coupling between excitons and longitudinal-optical (LO) phonons. As a result, the corresponding Huang-Rhys factors and LO-phonon energies are extracted for an elevated number of quantum dots. Finally, a microscopic parameter, known as the exciton-LO-phonon interaction volume was approximated for the first time, based on the presented detailed statistical analysis. Due to this extended, optical analysis of individual GaN quantum dots it was possible to characterize the optical traces of the biexciton cascade over a wide spectral range. Here, for a certain transitional range, a unique balance between one- and two-photon processes is observed, which arises from the biexciton decay and can be tuned means of temperature and excitation density. Especially the two-photon emission is a promising candidate for future applications as its temperature stability is demonstrated up to 50K. Interestingly, the particular case of biexcitonic complexes also forms the basis for the description of an entire new class of hybrid-quasiparticles with so far unknown spin configurations. An extended analysis of the optical properties of these hybrid-quasiparticles presents highly unconventional decay characteristics, demonstrating the outmost importance of the dark-excitons in hexagonal GaN quantum dots based on the present thesis

Maria Janitschek – eine vergessene Autorin der Jahrhundertwende? : Versuch eines werkbiographischen Portraits

Um die Jahrhundertwende galt Maria Janitschek als eine engagierte Autorin der Frauenbewegung im Kaiserreich. In ihren Werken griff die Österreicherin brisante Themen der Frauenbewegung auf: die sexuelle Erziehung von Mädchen, weibliches Sexualbegehren und Homosexualität. Janitscheks Werke riefen in der patriarchalischen Gesellschaft des Kaiserreiches heftige Reaktionen hervor, brachten ihr die Aufmerksamkeit der Frauenrechtlerinnen und den Unmut der Zensurbehörden ein. 1909 wurde ihr Werk „Die neue Eva“ (1902) verboten. Anders als Helene Böhlau oder Gabriele Reuter etwa hat sie jedoch in der Forschung nur wenig Beachtung gefunden. Abgesehen von zwei Dissertationen aus den 1950er Jahren über Leben und Werk sowie über ihre Lyrik ist keine umfangreiche Forschungsarbeit zu Maria Janitschek erschienen. Dieser Beitrag versucht Licht ins Dunkel der Janitschek Forschung zu bringen

Excited states of neutral donor bound excitons in GaN

We investigate the excited states of a neutral donor bound exciton (D0X) in bulk GaN by means of high-resolution, polychromatic photoluminescence excitation (PLE) spectroscopy. The optically most prominent donor in our sample is silicon accompanied by only a minor contribution of oxygen—the key for an unambiguous assignment of excited states. Consequently, we can observe a multitude of Si0X-related excitation channels with linewidths down to 200 μeV. Two groups of excitation channels are identified, belonging either to rotational-vibrational or electronic excited states of the hole in the Si0X complex. Such identification is achieved by modeling the excited states based on the equations of motion for a Kratzer potential, taking into account the particularly large anisotropy of effective hole masses in GaN. Furthermore, several ground- and excited states of the exciton-polaritons and the dominant bound exciton are observed in the photoluminescence (PL) and PLE spectra, facilitating an estimate of the associated complex binding energies. Our data clearly show that great care must be taken if only PL spectra of D0X centers in GaN are analyzed. Every PL feature we observe at higher emission energies with regard to the Si0X ground state corresponds to an excited state. Hence, any unambiguous peak identification renders PLE spectra highly valuable, as important spectral features are obscured in common PL spectra. Here, GaN represents a particular case among the wide-bandgap, wurtzite semiconductors, as comparably low localization energies for common D0X centers are usually paired with large emission linewidths and the prominent optical signature of exciton-polaritons, making the sole analysis of PL spectra a challenging task.EC/H2020/749565/EU/Heat Transport and its Effects on the Performance of Nanostructured, Photonic Materials/PhotoHeatEffectDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Hybridisation at the organic-metal interface: a surface-scientific analogue of H\"uckel's rule?

We demonstrate that cyclooctatetraene (COT) can be stabilised in different conformations when adsorbed on different noble-metal surfaces due to varying molecule-substrate interaction. While at first glance the behaviour seems to be in accordance with H\"uckel's rule, a theoretical analysis reveals no significant charge transfer. The driving mechanism for the conformational change is hybridisation at the organic-metal interface and does not necessitate any charge transfer.Comment: Accepted for publication in Chemical Communications. Main article: 6 pages, 2 figures; Supplementary Information: 4 pages, 3 figures, 1 table. All in one fil

Statistical Analysis of Atmospheric Models for the Performance of Re-entry Trajectory Simulations at Mars

Herr Callsen beschreibt in o.g. Masterarbeit einen innovativen Ansatz zur Schätzung von Unsicherheiten bei der Modellierung eines Wiedereintrittsvorgangs auf einen Körper mit Atmosphäre. Dieser Ansatz wird dann zur Untersuchung von Wiedereintrittsvorgängen speziell beim Mars angewandt. Hierbei benutzt Herr Callsen Theorien aus der Stochastik zur Untersuchung der zeitlichen Evolution einer Wahrscheinlichkeitsdichte. Ausgehend von initialen Wahrscheinlichkeitsdichten kann dann die Entwicklung der Wahrscheinlichkeitsdichten von Position und Geschwindigkeit während des Abstieges und insbesondere zum Zeitpunkt des Auftreffens auf der Oberfläche des Himmelskörpers berechnet werden. Unsicherheiten beim Wiedereintritt resultieren neben Ungenauigkeiten zum Anfangszeitpunkt beim Abkoppeln vom Trägerfahrzeug auch aus Unsicherheiten im Zustand der Atmosphäre hinsichtlich Gasdichte, Strömungsgeschwindigkeit usw., diese Größen können auch durch genaueste Modellrechnungen nicht erfasst werden, weil zu wenige Messdaten über sämtliche planetaren Atmosphären vorliegen. Herr Callsen benutzt in seiner Arbeit die Methode lineare Kovarianzanalyse und stellt die damit gewonnen Ergebnisse einer Monte-Carlo-Simulation gegenüber

Reseña : Espacio nómada en el ensayo autobiográfico del Accented Cinema

Obra ressenyada: Madalina STEFAN, Espacio nómada en el ensayo autobiográfico del Accented Cinema. New Orleans: University of New Orleans Press, 2021

Recombination dynamics in ZnO nanowires: Surfaces states versus mode quality factor

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 97, 133116 (2010) and may be found at https://doi.org/10.1063/1.3496444.In this work, we investigate the influence of finite size on the recombinations dynamics of ZnO nanowires. We demonstrate that diameter as well as length of nanowires determine the lifetime of the neutral donor bound excitons. Our findings suggest that while the length is mainly responsible for different mode quality factors of the cavity-like nanowires, the diameter determines the influence of surface states as alternative recombinations channels for the optical modes trapped in the nanocavity. In addition, comparing nanowires grown using different catalyst we show that the surfaces states strongly depend on each precursor characteristics.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeDFG, 53182490, EXC 314: Unifying Concepts in Catalysi